1. Field of the Invention
The present invention relates to a layer structured positive electrode material for a large lithium secondary battery that uses nonaqueous electrolytic solution, a positive plate, and to a lithium secondary battery. More specifically, the present invention relates to improvement of Li ion conductivity of a positive electrode material for a lithium secondary battery at low temperature.
2. Background Art
There has been demand for developing high power and high energy density batteries for power supply of electric automobiles and hybrid automobiles as environmentally friendly vehicles. Since lithium secondary batteries that use nonaqueous electrolytic solution have high battery voltage and high energy density as batteries used for power supply, various sectors have been industriously developing them. Also, batteries for automobiles are required to have long operating life, high power output, stable voltage controllability, environment resistance, and high safety, for example, in addition to the properties of conventional consumer batteries.
In order to extend the operating life of lithium ion secondary batteries, JP Patent Publication (Kokai) No. 2000-294240 A discloses a positive electrode material for trying to improve cycling characteristics. The material is represented by a composition of LiNixMn1—xO2 (0.05≦x≦0.3) resulting from LiNiO2 when Ni is substituted with Mn. In this case, layered lithium transition metal complex oxide, which is a positive electrode material, has a hexagonal crystal structure, and the crystal structure has a great influence on lithium ion insertion and extraction. Moreover, when the X-ray diffraction is carried out for the positive electrode material, the diffraction peak of a (003) plane is characteristic of a layered rock-salt structure, and the diffraction peak of a (104) plane is common to both the layered rock-salt structure and a cubic rock-salt structure. Thus, JP Patent Publication (Kokai) No. 2000-294240 A defines the proportion of the cubic rock-salt structure in the layered rock-salt structure in accordance with the diffraction peak intensity ratio of the (003) and (104) planes, and discloses the fact that the positive electrode material has superior cycling characteristics if the proportion is appropriate. Ni and Mn are in solid solution and form the cubic-rock salt structure (Mn) in layered Ni, which is a system that includes no Co. If this technology is applied to batteries for hybrid automobiles, the cycling characteristics are insufficient because of high Ni composition ratio in the positive electrode material, and the internal resistance of the batteries at −30° C. is high.
In contrast, JP Patent Publication (Kokai) No. 2002-151076 A discloses a positive electrode material represented by a composition of LiaNi1-b-cCObMncO2 (1.02≦a≦1.09, 0.05≦b≦0.35, 0.15≦c≦0.35, 0.25≦b+c≦0.55) that results in high density of battery capacity and superior cycling characteristics and heat stability by defining the peak intensity ratio (I(012)+I(006))/I(101)) upon X-ray diffraction, the average particle diameter, and the specific surface area of the positive electrode material. However, the aforementioned definition cannot be applied when the amount of Ni included in the transition metal of the positive electrode material is small, namely, not more than 45% in the atomic ratio, or when Ni is substituted with other transition metal whose ionic radius is different from that of Ni. Further, by the technology disclosed in JP Patent Publication (Kokai) No. 2002-151076 A, the amount of Ni included in the transition metal of the positive electrode material is high, namely, not less than 45% in the atomic ratio. If the aforementioned technology is applied to batteries for hybrid automobiles, when the content of Ni is large, cycle life is not sufficient since the crystal structure of the positive electrode material is unstable after a charge/discharge cycle. Also, the internal resistance of the batteries at −30° C. is high.
As mentioned above, the prior art cannot achieve both cycle life and the reduction of the internal resistance of batteries at −30° C.